Usually the notebook shuts down to protect the hardware because it’s overheating. This can be caused due dust as PauPau wrote and in such case you must clean your notebook with compressed air spray. This must be done every 3-4 months and it’s very easy procedure. Toshiba has released an article about this:http://forums.computers.toshiba-europe.com/forums/ann.jspa?annID=40

Do you use the preinstalled Vista version with all drivers from Toshiba or 3rd party drivers? 3rd party display drivers can also cause overheating…